Understanding the Cylinder Events in Four-Stroke and Two-Stroke Diesel Engines

Team Dmet Club
Apr 22
5 min read

Diesel engines play a critical role in marine, automotive, and industrial applications due to their efficiency, power output, and durability. Whether in a massive marine engine or a compact generator, the operation of a diesel engine relies on a sequence of events that occur within its cylinders.

These events vary slightly depending on whether the engine follows a four-stroke or two-stroke operating cycle.

This blog explores, in depth, the sequence of events that occur in both four-stroke and two-stroke diesel engine cylinders — presenting the science behind their mechanics and how each system achieves efficient power genera

tion.

🔧 Fundamental Requirements of a Diesel Engine

For any diesel engine to operate efficiently, five essential processes must take place:

Supply of air
Compression of air to increase temperature high enough to initiate fuel combustion
Injection of fuel into the hot compressed air
Expansion of hot, high-pressure gases to push the piston and produce power
Exhaust, or removal of combustion products from the cylinder

These stages may be distributed across either four strokes or two strokes of the piston — equating to two revolutions or one revolution of the crankshaft, respectively.

Source: Youtube

⚙️ Four-Stroke Cycle Diesel Engine

A four-stroke engine uses two valves (an inlet and an exhaust valve) operated by a camshaft. The cycle comprises four distinct strokes:

1️⃣ Intake Stroke

The cycle begins with the piston at Top Dead Centre (TDC).
The air inlet valve opens, and the piston moves downward.
This downward movement reduces cylinder pressure, drawing fresh air into the combustion chamber.
The high-velocity air entering through the inlet gains kinetic energy, known as the ram effect, which keeps the valve open even after the piston passes Bottom Dead Centre (BDC).
The inlet valve closes after the crank has moved 20° to 40° past BDC, once the kinetic energy is dissipated and airflow ceases.

2️⃣ Compression Stroke

The piston moves upward, compressing the air within the cylinder.
Depending on the engine design and speed, air pressure can range between 24 and 63 bars.
Fuel injection begins between 25° and 10° before TDC, initiating combustion shortly after.
Combustion continues as the piston moves past TDC, ensuring maximum pressure build-up.

3️⃣ Power (Expansion) Stroke

The combustion generates high-pressure gases (between 54 and 108 bars), which force the piston downward.
This movement rotates the crankshaft, delivering useful mechanical power.
As the piston nears BDC, the exhaust valve begins to open—typically at 50° to 40° before BDC—to initiate the blow-down period, during which the exhaust gases begin escaping.
The pressure inside the cylinder at this point reduces to around 3 to 4 bars.

4️⃣ Exhaust Stroke

The piston moves upward once more, expelling the remaining exhaust gases through the now fully open exhaust valve.
Because the valve is wide open, resistance to gas flow is minimal, allowing for efficient scavenging of combustion by-products.
Before the piston reaches TDC again, the air inlet valve starts to open, setting up the cylinder for the next cycle.

Note 1: Work is only performed during the expansion stroke. The intake and exhaust strokes consume energy, often sourced from the flywheel or other cylinders, resulting in pumping losses.

Note 2: The movement of valves is not instantaneous. There is a time lapse between the valve's initial opening and when it reaches its fully open position — this is governed by the design of the cam profile and the acceleration imparted to the valve.

⚙️ Two-Stroke Cycle Diesel Engine

Source: Youtube

Unlike the four-stroke engine, the two-stroke engine completes all five key processes in just two strokes — meaning every single revolution of the crankshaft generates a power stroke. It achieves this with the help of ports (not valves) and scavenger air, a low-pressure airflow system.

🔄 Combined Exhaust and Intake Stroke

As the crank rotates from 45° to 40° before BDC, the piston uncovers the exhaust ports, releasing combustion gases into the exhaust manifold (blow-down phase).
Simultaneously or shortly after, the piston uncovers the scavenger ports at around 35° before BDC.
Scavenger air, supplied at a pressure of 0.06 to 0.25 bars, enters the cylinder and pushes out residual exhaust gases — a process essential for cylinder cleaning and fresh air charging.
The scavenger air supply is cut off around 35° past BDC, after which both ports start getting covered by the upward-moving piston.

🔁 Compression and Power Stroke

Once the ports are closed, the compression phase begins — similar to that in a four-stroke engine.
Fuel is injected near the TDC, followed by combustion, which forces the piston downward, thus performing useful work.
The cycle repeats with the next exposure of exhaust and scavenger ports.

The entire cycle of intake, compression, combustion, expansion, and exhaust occurs in just two strokes — a feat achieved by careful timing and port geometry.

Important Note: The maximum pressure in two-stroke engines is lower than in four-stroke engines, primarily due to overlapping port operations and reduced time for effective compression.

🆚 Key Differences at a Glance

Aspect	Four-Stroke Engine	Two-Stroke Engine
Power stroke frequency	Once every 2 revolutions	Once every revolution
Valve/Port System	Uses valves (inlet & exhaust)	Uses ports (scavenge & exhaust)
Scavenging	Not required	Required, done using scavenger air
Mechanical Complexity	More complex (valves, cams, rocker arms)	Simpler design (ports only)
Efficiency	Higher thermal efficiency	Higher power-to-weight ratio
Pumping Losses	Present during intake and exhaust strokes	Minimal pumping losses
Maximum cylinder pressure	54–108 bars	Lower due to shorter compression phase

📝 Conclusion

Both four-stroke and two-stroke diesel engines have their distinct design philosophies, advantages, and limitations. The four-stroke engine provides better fuel efficiency and cleaner operation, making it ideal for most industrial and automotive applications.

On the other hand, the two-stroke engine offers a higher power-to-weight ratio and simpler construction, which is why it continues to be preferred in certain marine and heavy-duty operations.

Understanding the detailed sequence of events in each type of engine helps us appreciate the engineering ingenuity that powers our world — one piston stroke at a time.

📌 Disclaimer

This blog has been curated and compiled with valuable contributions from DMETians and DMECA members, along with extensive support from AI platforms and detailed internet research conducted by our content team. Every effort has been made to ensure the accuracy, clarity, and technical correctness of the information presented. However, if you notice any inadvertent errors or omissions, we sincerely apologise.